You're probably will have to take the bumper off and either do as Jon suggested, or use a HEAVY piece of metal behind the bend/dent and use a shop hammer to flatten the bend to the back dolly (metal) I used a 50 lb. anvil to straighten my Thing bumper. Smaller dents can be beat out using a body shop dolly as the backing. The problem is getting into position to hit the bend/dent with the bumper bolted to the vehicle. In any case, the bumper needs to be between a hammer and an immovable hard place. Some times it can be done by putting the bumper on a piece of wood on a cement step/sidewalk/garage floor etc. and beat the slop out of it there._________________Ya can't have too many VDUBS.
1956 Oval
1966 Beetle
74 Thing (in progress)
74 SB
76 Westy P22 (Arizona Bus)